1. Field of the Invention
The present invention describes a method and system to produce highly purified methanol from a methanol condensate wherein a preferred embodiment the methanol condensate is a biomethanol from biomass sources.
2. Description of the Prior Art
Chemical pulp mills, including Kraft mills generate considerable amounts of condensates during the pulp-making process. Condensates contain several undesirable compounds that make their recycle and reuse impossible without treatment. Condensates are generally generated in the black liquor evaporation and digester areas. Methanol condensates can also be obtained from other sources such as pulp and municipal wastes.
It has been reported that condensates generated at kraft pulp mills contain several volatile organic compounds (VOCs), total reduced sulphur (TRS) compounds, and traces of black liquor. More than 150 compounds have been detected in foul condensates from kraft pulp mills (Niemela, K., Sulfur and Nitrogen Compounds in Rectified Methanol from Foul Condensate Stripping, PAPTAC/TAPPI International Chemical Recovery Conference Proceedings, Charleston, S.C., Jun. 6-11, 2004). The main total reduced sulphur (TRS) compounds reported are hydrogen sulphide (H2S), methyl mercaptan (CH3SH), dimethyl sulphide (CH3SCH3), and dimethyl disulphide (CH3SSCH3). The TRS compounds have an offensive odour and their direct discharge to the ambient air or to an effluent treatment system can raise odour concerns from the communities near the mill and/or may cause the mill to exceed its emission limits with respect to these compounds. Other VOCs include methanol, ethanol, acetone, and terpenes. Methanol is by far the major VOC in kraft pulp mill condensates.
The major foul condensate treatment methods at kraft pulp mills include air and steam stripping. Air stripping removes only TRS compounds and requires an air to condensate ratio of 3-5% on a mass basis. The gaseous stream is then burned in a boiler, kiln, or an incinerator. The TRS removal efficiency is a function of temperature and pH of the feed solution. Generally, this approach removes more than 90% of the TRS compounds present in the condensate. Only a few mills in North America air strip their condensates.
Steam stripping is the dominant condensate treatment approach in the pulp and paper industry. To remove most of the TRS compounds and most of the methanol in the feed stream, a steam to condensate ratio of 15-20% on a mass basis is required. The TRS compounds, the turpentine (softwood pulp mills) and methanol are concentrated in the stripper-off gases (SOG). These gases are commonly burned in the recovery boiler, power boiler, lime kiln, or in a dedicated incinerator.
Most of the methanol on the market is produced from natural gas by steam reforming. During this process, natural gas is first converted into synthesis gas consisting of CO, CO2, H2O, and H2. These gases are then catalytically converted to methanol. In fact, methanol can be produced from any resource that can be converted to synthesis gas such as biomass, agriculture residue, municipal and industrial wastes, and other feedstocks. These steps take place at high temperatures and pressures. The crude product containing methanol, water, and other high boilers is purified by distillation. Sodium hydroxide can be added to reduce carbon steel corrosion and avoid an expensive metallurgy. The sodium hydroxide reacts with organic acids present in the raw methanol and reduces their concentration levels in the methanol final products.
Several patents such as U.S. Pat. Nos. 5,063,250, 5,387,322, 6,214,176, describe the distillation of raw methanol obtained from methane steam reforming.
U.S. Pat. No. 5,863,391 describes a method to purify methanol by removing acetaldehyde using extractive distillation. Ethyl glycol was used in this case to enhance the volatility of acetaldehyde.
A significant amount of methanol can be present in the foul condensate at kraft pulp mills which ends up in the stripper-off gas. Few attempts have been made to recover or to further purify the methanol content of these streams for use within the mill or for sale for specific applications.
US Patent Application 2011/0306807 A1 described a method of producing methanol from SOG condensate. The SOG condensate is decanted to remove terpenes and then two distillation columns are employed to purify the methanol. FIG. 1 presents the layout of the process as implemented at a kraft pulp mill (Jensen, A., Ip, T. and Percy, J. Methanol Purification System, 2012 TAPPI PEERS Conference, Savannah, Oct. 14-18, 2012, page 1245). High purity methanol (99.85%) was the target of this process. However, the presence of several contaminants such as sulfur and nitrogen species made it difficult to reach a final methanol product that is suitable for sale and meets the International Methanol Producers and Consumers Association (IMPCA) standards. Further treatment of the condensate and/or the final product is required to achieve the desired methanol quality suitable for sale.